Condenser for soil moisture meter



Dec. 8, 1942. J. E. FLETCHER CONDENSER FOR SOIL MOISTURE METER Fil edJuly 19, 1940 2 Sheets-Sheet l ,I JVR NW TKIHCQM FIG.

INVENTOR Joe/ E. Fla/char ATTORNEYS Dee. 8 3.9420

J. E. FLETCHER Filed July 19. 1940 FIG. 4

PER

2 Sheets-Sheet 2 INVENTOR Joe/ E, F/eic/zer ATTORNEYS latented Dec. 8,1942 UNITED STATES PATENT OFFICE (Granted under the act of March 3,1883, as amended April 30, 1928; 370 0. G. 757) 7 Claims.

This application is made under the act of March 3. 1883, as amended bythe act of April 30, 1928, and the invention herein described andclaimed, if patented, may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment to me of any royalty thereon.

This invention relates to a device for electrically measuring themoisture content of soil and is more particularly concerned with thecondenser unit thereof which is adapted to be sunk or imbedded in theground where the moisture determination is to be made.

In the accompanying drawings .there are illustrated two generalembodiments of my invention The first embodiment, referred to generallyas the plate type, is illustrated in Figures 1, 2, and 3; Figure 1 beinga front elevational view, Figure 2 a section along the line 2-2 ofFigure 1, and Figure 3 a section along the line 3-3 of Figure 1.

The second embodiment, referred to generally as the cylindrical type, isillustrated in Figures 4, 5, and 6.; Figure 4 being a longitudinalsection, Figure 5 being a cross section along the line 55 of Figure 4,and Figure 6 being a fragmentary elevational view, portions of thedielectric material being broken away to show the helical arrangement ofthe apertures in the outer cylindrical plate.

Referring with more particularity to Figures 1, 2, and 3, the embodimentillustrated comprises a pair of copper plates H), which may be of heavyfoil, disposed in fixed spaced relation and covered with a coat I l oflead glass, Lucite, or other suitable material. Between the coatedplates 10, a water-absorbent dielectric material I2, such as plaster ofParis, or other suitable material, is disposed. This dielectric materiall2 also serves to hold the plates I 0 in position. A long hollow pipe I3is bifurcated at one end to form two forks I4, between which the coatedplates I0, together with the dielectrical material [2, as a unit, isdisposed and secured in place by means of bolt and nut assemblies l5,substantially as shown. Wire leads 16, of shielded microphone cable areconnected to the plates 18 at one end; the shield being fastened to oneplate and the center lead to the other plate. The other ends of theleads I6 are threaded through the hollow core of the pipe l3 andconnected to suitable tip jacks (not shown).

The embodiment illustrated in Figures 4, 5, and 6, comprises a metalbase I! to the bottom 0! which is removably secured a spear tip l8.Adjacent the tip 18, the base I! is recessed to provide a cylindricalcore l9 about which there is first disposed a coated cylindrical collar20 of copper or other suitable material, the coat 2| thereof beingpreferably of Lucite or lead glass. Another copper cylinder 22 isdisposed concentric with the cylinder 20 and likewise covered with acoat 23 of Lucite, lead glass, or other suitable material. The cylinder22 is larger than the cylinder 20 and the space between their twocoatings is filledwith plaster of Paris 24, or other suitablewater-absorbent dielectric material. The outer cylinder 22 is providedwith apertures 25 in helical relation. These apertures are also filledwith the dielectric material used between the two cylinders 20 and 22,and on the outside of the cylinder 22 a coating 26 of the samedielectric material is provided, The member I1 is provided with alongitudinal bore 21 through which electrical shielded wire leads 28 aredisposed and connected to the cylindrical plates 20 and 22 in the samemanner as above described with reference to the first embodimentillustrated in Figures 1, 2, and 3. The upper end 29 of the member 1! isthreaded for a removable connection to a suitable pole or rod 30 so thatthe device can be inserted in the ground at desired depths, ashereinafter explained.

To use either of the two embodiments abovedescribed, a hole is drilleddiagonally into the soil to the desired depth with an auger, or othersuitable tool, having the same diameter as the condenser unit. Thecondenser unit is then forced firmly against the bottom and sides of thehole. It is necessary for the exposed portions of the plaster of Paristo press firmly against the soil on the sides of the hole to make a goodcontact for the passage of the moisture. The sides or the hole should bepacked firmly all around and the remainder of the hole filled with soiland packed tight to prevent water from following the pole or rod to theplates. The wire leads from the condenser plates are then connected to ameter for measuring capacity. The capacity of the buried condenservaries with the dielectric constant, which in turn varies with themoisture content in the dielectric material. This moisture content isafiected by the moisture of the surrounding soil. The lower the moisturecontent of the dielectric material the smaller is the value of thedielectric constant and, consequently, the capacity of the condenser islowered. Thus, the capacity of the condenser becomes a function of thepercentage of moisture in the soil.

Having thus described my invention, I claim:

1. A condenser for a soil moisture meter comprising a pair of metalplates disposed in fixed spaced relation, a coating of lead glass abouteach of the said plates, a plaster of Paris dielectric permanently fixedbetween said plates; said dielectric being communicable with the soil tobe tested.

2. An article of manufacture comprising a rod, a condenser secured atone end of said rod; said condenser comprising a plurality of spacedmetal plates, a coating of insulation about each of said plates, awater-absorbent dielectric permanently fixed between said plates, andelectrical wire leads attached to said plates for connection to acapacity meter.

3. An article of manufacture comprising a rod having a spear point atone end and a cylindrical recessed portion adjacent said spear point, aninsulated metal cylinder disposed about said recessed portion, a layerof water-absorbent dielectric material about said cylinder, an insulatedforaminous metal cylinder about said dielectric material, and anotherlayer of water-absorbent material about the exterior of said foraminouscylinder contacting the dielectric material between said cylinderthrough the perforations of said foraminous cylinder.

4. An article of manufacture as defined by claim 3 in which theperforations of the forminous metal cylinder are disposed in helicalrelation.

5. An article of manufacture comprising a rod having a spear point atone end and a cylindrical recessed portion adjacent said spear point, aninsulated metal cylinder disposed about said recessed portion, a layerof plaster of Paris about said cylinder, a perforated and insulatedmetal cylinder about said layer of plaster of Paris, another layer ofplaster of Paris about said peri'orated cylinder; said layers of plasterof Paris being in contact with each other through the perforations ofsaid perforated cylinder.

6. A condenser for a soil moisture meter comprising a pair of metalplates disposed in fixed spaced relation, a coating of insulation abouteach of said plates, a dielectric material permanently fixed betweensaid plates, said dielectric being communicable with soil to be testedand capable of absorbing moisture from said soil.

'1. A device of the character described comprising a condenser havingelectrode plates, a moisture shield surrounding each of said plates,

- and a permanent dielectric material between said plates capable ofabsorbing moisture from adiacent soil, the dielectric constant of saidmaterial varying with the quantity of moisture absorbed.

JOEL E. FLETCHER.

